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Hess's Law Main Rule

Hess's Law of Constant Heat Summation (or just Hess's Law) states that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes. This law is a manifestation that enthalpy is a state function. (chemwiki)

Intermediate

DONT FORGET

Does not matter which route you take; the end product will ALWAYS be the SAME! 

Intermediates

The Intermediates refer to the components in the reaction which are halfway between reactants and  being converted into the products.

Law of Conservation of Energy

The Enthalpy change of a reaction is dependent on the difference between the enthalpy of the products and the enthalpy of the reactants.

 

Enthalpy Change of a reaction is the same despite the route which is taken. It can go directly to the product or through an intermediate

 

This law refers to the law of conservation of energy.

Law of enthalpy of combustion can be used to find out enthalpy changes that can't be measured directly.

 

 

Law of conservation of energy : DIAGRAM Diagram A

Law of conservation of energy : EXAMPLE

Enthalpy of combustion of both carbon and carbon monoxide can be measrued directly, combustion of carbon to carbon monoxide cannot be measured directly.

 

(diagram A)

To find out delta H of reaction add up all enthalpy values

if arrow is facing wrong way flip it and change sign of delta H

Solving by Simultaneous Equation 

Write out known equations and then manipulate them to get required equation.

 

1) C(s)+ O2(g) ----> CO2(g)      delta H= -394 kJ mol-1

2) CO(g) + 1/2 O2(g) ---> CO2(g) delta H= -283 kJ mol-1

 

Subtract 2 from 1

 

C(s) + 1/2 O2 (g) - CO (g) = 0 delta H = -394 - (-283) kJ mol-1

 

Rearrange equation

 

C(s) + 1/2 02(g) ----> CO delta H = -111 kJ mol -1

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